1. Field of the Invention
The present invention relates to a system for transferring wafers in semiconductor fabricating equipment. More particularly, it relates to a wafer transfer system including a serial number detecting device that recognizes wafer numbers marked on the wafers and automatically arranges the wafers in a predetermined order (e.g., an ascending order, a descending order, an odd/even number order or an individual selection order).
2. Background of the Related Art
Semiconductor fabricating equipment has undergone continued remarkable development and has entered an era of intricate, various and automated functioning capable of advanced control. Complex semiconductor fabrication processes are now possible. It is the semiconductor fabricating equipment which determines the properties of the semiconductor products; and high technology fabrication depends on the quality of the semiconductor fabrication equipment.
Therefore, reliability of the equipment is necessary in order to thoroughly perform hundreds of processes for fabricating semiconductor devices under designated conditions. The processes have respective unique parameters and procedures. Numerous factors that may affect the process condition can be generated while semiconductor devices move through these many processes.
Each unit process among the hundreds of processes is carried out, not independently, but in relation to each other unit process in order to produce a finished product. Upon completion of a unit process, a test step is provided to ascertain whether or not the preceding unit process was normally performed. Thereafter, the results from the test are verified. In such a manner, the unit processes are continuously performed. In this case, the reliability of the equipment has much more importance because a highly integrated semiconductor device is being fabricated.
The importance of reliability will be described in relation to a dry-etching process, which is widely used in semiconductor fabrication. For description purposes, two control points are designated. The two control points are input factors and output factors. Based on these control points, the influence that the equipment has on semiconductor fabrication will be discussed below. The technique using these two control points is known as statistical process control. The input factors and the output factors are shown in Table 1.
TABLE 1 __________________________________________________________________________ Input Factors and the Output Factors for Semiconductor Fabrication __________________________________________________________________________ Input Factors Equipment Gas, Temperature, Pressure, Highly Influenced by High-frequency Wave, Etc. the Mechanism Environment Temperature, Purity, Humidity, Etc. Method Process Sequence, Time in an Etching Chamber, Etc. Output Factors Test Critical Dimension, Etch Rate, Results from Process Dissemination, Process Verification Defects, Profile State, Etc. __________________________________________________________________________
In Table 1, the input factors represent the mechanical aspect of the equipment for performing dry etching; and the output factors represent the properties of a product (e.g., a patterned film on a wafer) produced by the dry etching process.
Referring to Table 1, it is noted that the output factors, that is, critical dimension, the etch rate, etc. of the patterned film, are determined by the process condition of the input factors. In the event that reliability of the equipment is not achieved, the result is deterioration of the output factors.
FIG. 1 shows the relationship between time and film growth when the film grows on a plurality of wafers in a batch technique. N.sub.1 through N.sub.x represent the respective wafer numbers.
Referring to the graph of FIG. 1, the film initially grows at the velocity of V.sub.1 on the wafer N.sub.1. As time elapses, the film grows on the wafer N.sub.x, and the temperature in a reaction chamber in which the film grows on the wafers is gradually increased in association with RF (Radio Frequency) heating. As a result, the velocity V.sub.1 is changed by .DELTA.V. The resultant velocity is shown as V.sub.2 in the drawing. The change in the velocity of the film growth causes differences between the thicknesses of the resultant films on the wafers, which results in process degradation.
Upon completion of a unit process, the wafer is subjected to a test step. If the test shows that the thickness of the film is not in the range of a reference thickness (e.g., a set value .+-.0.5%), the wafer having the film is rejected. Otherwise, if the thickness is in the range of the reference thickness, the wafer is transferred to the next process in the sequence of processes.
When the wafer has a film thickness that is not the same as the set value but is in the range of the reference thickness, the wafer is transferred to the next process anyway. In this case, however, the wafer may encounter difficulties later on in the fabrication process. In particular, if the wafer is rejected during any test following a subsequent unit process, it is difficult to identify which equipment or which process caused the degradation of the quality of the wafer which lead to the rejection.
That is why wafers are randomly processed and loaded into a cassette without any designated order (e.g., an order based on the wafer numbers marked on the wafers). Rather, the wafers are loaded into the cassette when they have completed a respective unit process.
Hundreds of processes using batch techniques and single techniques (i.e., one by one) make the wafer processing order much more random. The batch technique is generally applied to, for example, a process using a diffusion furnace. The single technique is used, for example, for a dry etching process or an ion implantation process. A CVD (Chemical Vapor Deposition) process uses a partial batch technique. In the partial batch technique, wafers are introduced into the fabricating equipment in the batch manner, while they are processed in the single manner.
When the wafers are randomly loaded into the cassette, it is impossible to determine a relationship between the causes of the process degradation that are generated during the respective unit processes and the input factors of the fabricating equipment using only the poor quality wafer.
Therefore, further research and development for solving the above and other problems is required to thoroughly understand the relationship between the input factors, i.e., the properties of the equipment, and the output factors, i.e., the properties of the products.